What do your Fins Do?

Surfboard fins are wonderful, beautiful objects. Works of art have been made from them, and we’re told they can do amazing things to the performance of our surfboards. Hours can be spent trying to decide between different sizes, shapes and flex patterns, and companies like FCS and Futures spend huge amounts of money developing and testing new designs, as we try to get the best performance we can from our boards. But what do they actually do?

A post shared by Harry Knight (@hjmknight) on May 8, 2015 at 1:33pm PDT

Very simply, the fins on your board have 3 basic jobs to perform:

Provide Lateral Resistance to keep the board moving in a straight line (like the keel on a boat).

Help keep the board’s rail locked into the wave face, allowing it to create forwards thrust (see “How a Surfboard Works).

Effect the turning characteristics of the surfboard: either tightening or drawing out the turn radius.

Now, what I’m not going to do in this article is talk about the shape of surfboard fins. There’s a whole ton of articles that explain the various effects of different fin shapes such as these ones from Tactics and Board Cave. What I want to talk about instead, is how the fins interact with the water flow around them, and how that affects the board, as without this information it’s very hard to fully understand how changing your fins will affect the board.

A good illustration of how different aspects of a fin will affect the performance of a surfboard.

The first thing to note is that fins are totally optional on any surfboard. You can paddle into a wave, surf down the line, and even perform simple manoeuvres without any need of a fin. However there are certain manoeuvres you cannot do without fins, and pretty much everything is made easier by using them, unless you’re Julian Wilson here on a finless CatchSurf Beater.

For the basis of this discussion, we’re going to separate fins into 2 different categories: fins that sit on, or close to, the centre line of the board, and fins that sit on, or close to, the rails.

Centreline Fins (L) and Rail Fins (R)

Fins that sit on the centre line of the board are primarily there to prevent the tail from sliding sideways, especially at low speeds. They are most effective at this if they’re as close to the tail as possible; that way, if the tail of the board tries to drift, the fin’s surface area can provide Lateral Resistance. This helps to keep the board moving forwards, but also produces a lot of drag which can slow the board down. Centre fins also help to provide a central pivot point around which the board can rotate, so boards without a centre fin can sometimes skid in turns.

Fins that sit on the rail line are a little more complicated. While they do provide lateral resistance, They’re often less effective at this as they normally sit further forwards on the board. Their main function is to generate hydrodynamic lift, in the same way as an aeroplane wing does, which acts at 90° to the fins surface and helps to suck the surfboard into the wave face. Going back to my previous article on how surfboards work, you can see how sucking the rail into the wave will help the board re-direct more water, resulting in more forwards speed. This extra grip also allows the rider to push harder on the rail as they pump down the line, resulting in even more speed.

Now, when choosing a fin set-up, the first thing to think about is the relationship between the fins and the outline of the board. For example, once travelling across the wave, a longboard with a long, straight outline, already has a large amount of rail engaged in the wave face. Extra grip from side fins is probably not necessary, and may even make turning harder when you try to extract that rail from the wave. What you will need however, is a solid pivot point to push against when you try to break the rail out, and this is why most longboards carry a large single fin. Many longboards also have the option of side fins, and these might be useful in steeper or more powerful waves, when the extra grip could make all the difference.

The long, straight rail line of a longboard will grip the wave well, while the shorter, more curved rail of a shortboard might need some help from the fins.

On a shorter and/or fatter board, it’s not possible to get so much rail in the wave face, and so the side fins become more and more important. In some designs, particularly short, stubby boards, the centre fin is often taken out all together, and the board is ridden with just the side fins (Twins, Quads, Twinzers, etc) to give it as much speed as possible.

Boards with both side and centre fins (Thrusters, Bonzers, Widowmakers, etc.) offer a nice balance between the suction and speed of the side fins and the hold and pivot of the back fin, and the 3 fin Thruster has probably been the most common fin set-up found on surfboards for the last 30 years.

The final piece to this puzzle was the introduction of removable fins, and the ability to swap set-ups to suit the conditions. Many boards now come with 5 fin boxes, allowing the rider to ride a 3 fin Thruster for vertical top to bottom surfing, then switch over to a quad or twin fin set up, for fast, down-the-line surfing.

This animation shows just some of the fin combinations you can use on a 5 fin board.

Hopefully this information can help you to look at the fins on different boards in a slightly different light, and to understand what putting larger or smaller fins in different positions will do to the way a board handles. That will in turn give you some better insight when you’re looking at all the different fin templates for your new board.

Harry, I love what you and the SurfSimply guys do (Podcast addict). I’m a Design Engineer by trade Beginner Surfer by wave, and love the technical / scientific approach that you guys use to formulate your advice and conclusions. Its refreshing to hear and read surf terminology broken down into information that I can digest and apply to my wave riding. I have improved significantly just from listening to you guys and reading your work.

I moved from Bristol to Perth, WA so its also just nice to listen to some friendly English chaps talk! (Ash is awesome too)

Just wanted to say…Thank you and the team for producing excellent material, and keep up the good work.